1. Carrier Aggregation Technique
Peak rate of Long Term Evolution Advanced (LTE-A) has been dramatically increased with respect to that of Long Term Evolution (LTE). In LTE-A, a downlink rate of 1 Gbps and an uplink rate of 500 Mbps are required. At the same time, LTE-A systems are required to be compatible with LTE systems. In the consideration of improving the peak rate, compatibility with the LTE systems and the utilization of the spectrum, a Carrier Aggregation (CA) technique is proposed in the LTE-A systems.
The CA technique refers to a scheme in which a User Equipment (UE) may aggregate multiple cells and the multiple cells may provide a data transmission service to the UE at the same time. In a CA system, carriers corresponding to the cells may be successive or not in the aspect of frequency. In order to be compatible with the LTE system, a maximum bandwidth of each component carrier is 20 MHz. Bandwidths of the component carriers may be the same or different.
In the CA system, cells in which the UE works are divided into a Primary Cell (PCell) and several Secondary Cells (SCells). The PCell is responsible for most control and signaling works (e.g., random access, transmission of uplink feedback to downlink data, report of Channel Quality Indicator (CQI), transmission of uplink pilot, etc.). The SCell mainly acts as resources to bear data transmissions.
2. Random Access Scheme in the LTE System
The random access in the LTE system includes a contention-free random access and a contention-based random access. The procedure of the contention-free random access is as shown in FIG. 1, which includes the following operations.
Msg (Message) 0: an eNB allocates for the UE a Random Access Preamble Index (ra-Preamblelndex) used for the contention-free random access and a Random Access Physical Random Access Channel (PRACH) Mask Index (ra-PRACH-Masklndex) used for random access. For a contention-free random access initiated by arrival of downlink data, a Physical Downlink Control Channel (PDCCH) is used for carrying the above information. For a contention-free random access initiated by handover, a Radio Resource Control (RRC) signaling is used for carrying the above information.
Msg1: according to the ra-Preamblelndex and ra-PRACH-MaskIndex indicated in the Msg0, the UE transmits on the dedicated PRACH resource a designated dedicated preamble to the eNB. After receiving the Msg1, the eNB calculates an uplink Timing Advance (TA) value according to the Msg1.
Msg2: the eNB transmits a random access response Msg2 to the UE, wherein the random access response Msg2 carries timing advance information and UL grant resource allocated for subsequent uplink transmission. The timing advance is used for determining the timing relationship of a subsequent uplink transmission of the UE. The PDCCH carrying the Msg2 is scrambled by RA-Random Radio Network Temporary Identity (RA-RNTI). The RA-RNTI uniquely corresponds to a time-frequency resource in which the Msg1 is transmitted within a 10 ms window. In addition, the Msg2 also carries a preamble ID. The UE determines that the Msg2 is corresponding to the Msg1 transmitted by the UE according to the RA-RNTI and the preamble ID.
The procedure of the contention-based random access is as shown in FIG. 2, which includes the following operations.
Msg1: the UE selects a random access preamble and a PRACH resource, and transmits the selected random access preamble to the eNB using the PRACH resource.
Msg2: after receiving the preamble, the eNB calculates a timing advance (TA) value and transmits a random access response to the UE, wherein the random access response includes at least timing advance information and a UL grant for Msg3.
Msg3: the UE performs uplink transmission on the UL grant designated by the Msg2. For different random access reasons, the Msg3 transmits different contents in uplink. For example, for an initial access, the Msg3 transmits an RRC connection establishment request.
Msg4: a contention resolution message. The UE may determine whether the random access succeeds according to the Msg4.
The eNB may also use the Msg0 to initiate the contention-based random access. Different from the contention-free random access, the eNB indicates the UE to initiate the random access but does not indicate the detailed random access resource.
Based on the above procedure, the UE transmits uplink data and Hybrid Auto Repeat Request (HARQ) feedback information to downlink data according to requirements of the uplink timing advance. Thus, the eNB can receive the uplink transmission at a desired time point, which is referred to as uplink synchronization.
3. Multiple Timing Advance (Multi-TA) Scenarios Defined by CA
Due to the involvement of the CA, if frequency characteristics and transceiver distances of cells working on different carriers have a large difference, there may be different uplink timing advances for different carriers. Currently, 3GPP defines two scenarios supporting multi-TA. For example, FIG. 3 is a schematic diagram illustrating a scenario involving a Radio Remote Unit (RRU). For example, a large coverage (large circle) is provided by use of Frequency (F) 1, a Remote Radio Head (RRH) provides a hot spot coverage (small circle) by use of the F2 within the coverage of a F1 cell. Mobility managements are performed based on the F1. In this scenario, if the UE is located in an area overlapped by the F2 cell and the F1 cell, the F1 cell and the F2 cell may be aggregated. But the UL TAs of the F1 cell and the RRH cell are different.
FIG. 4 is a schematic diagram illustrating a scenario in which a repeater is involved. For example, the eNB supports the F1 and the F2, wherein the eNB provides a large coverage by use of the F1 (filled with left bias lines) and the eNB provides a small coverage by use of the F2 (filled with right bias lines). Through a frequency selective repeater, the coverage area of the F2 may be enlarged. In this scenario, if the UE is located in an overlapped area of an F1 cell and an F2 cell, the F1 cell and the F2 cell may be aggregated. But the UL TAs of the F1 cell and the F2 cell are different.
In conventional systems, in order to facilitate maintenance of the TAs of the Multi-TA system, a concept of a TA group is proposed. The UE in the cells belonging to the same TA group may use the same TA value for their UL Component Carriers (CCs). The UE in the cells belonging to different TA groups uses different TAs for their UL CCs. Within one TA group, the UE only needs to keep uplink synchronization with one cell to realize uplink synchronization with all cells in the TA group.
During the implementation of the present invention, the inventor finds that the conventional system has at least the following problems.
The current protocol supports only random access into the PCell. The random access procedure into the PCell is the same as that of a single cell in previous releases. If there are multiple TAs, the timing advance of the PCell may be different from those of other cells, the current random access procedure cannot meet the requirements of obtaining and maintenance of multiple uplink timing advances.